Asymmetric digital subscriber lines (ADSLs) use standard phone lines to deliver high speed data communications by utilizing a portion of a phone line's bandwidth that is not utilized by voice, allowing for simultaneous voice and data transmission. The data transmission over an ADSL is continuously available, and ADSL technology helps to bring full-motion video, telecommuting and high-speed Internet access to a home or business. The line is termed “asymmetric” because it uses most of the channel to transmit downstream to the user, and only a small portion of the channel to receive upstream information from the user. A typical ADSL may deliver upstream at speeds of approximately 16-768 kilobytes per second (kbps) and downstream at speeds of approximately 1.5-10 megabytes per second (Mbps).
Since the origin of ADSL, improvements have been made in the technology in areas such as efficiency, affordability and functionality, resulting in the adoption of new standards. ADSL2 standards, completed and approved by the International Telecommunications Union (ITU) in 2002, supersede existing ADSL standards. Additionally, ADSL2+, approved by the ITU in January 2003, doubles the downstream bandwidth of ADSL2 to as much as approximately 25 Mbps.
Loop bonding technology provides the telecommunications industry with a revolutionary technique for combining multiple data communication lines, such as ADSL2 and ADSL2+ connections, together into a single, aggregated connection, even if the multiple connections have different capacities. In January 2005, the ITU passed the G.998.1 (G.Bond-ATM) standard. G.Bond-ATM enables ADSL equipment to electronically bond together ADSL2 or ADSL2+ links using multiple copper phone lines, which can then be used to dramatically increase the bandwidth provisioned to subscribers, via a single asynchronous transmission mode (ATM) data stream, to approximately 45 Mbps. These ultra-high data rates support advanced services such as broadcast video and video on demand for customers otherwise out of reach because of their distance from the central office or digital loop carrier (DLC). Even multiple streams of high-definition television can be supported using bonded ADSL2+. The bonding, or other type of bundling, may be automated using software.
Disparate data rates may be supported among the bundled of lines, for example, up to a ratio of 4:1. If some lines have lower capacity than others, it is not necessary to reduce the data rates on the other lines.
However, along with the benefits resulting from the ability to bundle and support disparate data rates among the set of bundled data communication lines come several disadvantages. Cells are assigned to lines in an order in accordance with their associated SIDs. However, the SID difference between simultaneously transmitted cells on different ADSLs may increase due to the different data transmission speeds of the different lines. Buffers in a transmit node, corresponding to the ADSLs, that accept multiple cells and operate using a simple round robin cell assignment technique, may result in large SID differences. This is due to the fact that buffers associated with slow data communication lines may not be restricted from accepting cells with consecutively numbered SIDs. A reorganization procedure at a receive node is initiated because ATM cells are received in a different sequence than that in which they were assigned to the lines, due to the differing line speeds. Thus, those cells arriving before their proper placement in the sequence, based on their SID, are buffered in a memory until the proper position in the sequence is reached. The reorganization procedure must wait for the longest delayed cell that is out of order.
Accordingly, what is needed is an improved approach to minimizing SID difference in simultaneously transmitted cells resulting from cell transmission delay in a set of bundled data communication lines.